Electrically isolated heat sink lead frame for plastic encapsulated semiconductor assemblies

Abstract

An improved lead frame having an electrically isolated heat sink portion is provided for the manufacture of high heat dissipating semiconductor devices. A large heat sink metallic portion having an exposed surface has plastic encapsulating material partially disposed thereabout such that associated semiconductor device leads are correctly positioned with respect to the heat sink metallic portion but are electrically isolated from it. A semiconductor chip is mounted directly on the heat sink metallic portion such that high heat dissipation is obtained. The lead frame provides for a plurality of metallic heat sink portions joined to a corresponding plurality of semiconductor device lead portions. Joining means positioned in the frame between adjacent metallic heat sink portions and correspondingly between successive semiconductor device lead portions are attached to each other to hold the respective portions of the lead frame in proper relationship to each other during the assembly process. These joining means are removed after plastic encapsulation thereby making it possible to electrically isolate the metallic heat sink portion from the semiconductor device lead portion. The use of this type composite lead frame structure where the metallic heat sink portion is joined to the device lead frame structure allows great versatility in types of material and types of fabrication steps used to provide a semiconductor device while the particular joining means used make it possible to obtain these advantages while still maintaining an electrically isolated heat sink.

What is claimed is: 
     
       1. A process for the automated manufacture of semiconductor devices characterized by the steps of: (a) forming a first preassembly structure comprising a plurality of die mount portions and first joining means interposed between said die mount portions;   (b) forming a second preassembly structure comprising groups of separated lead members and second joining means interspersed between said groups;   (c) forming a composite lead frame by joining said first and said second joining means;   (d) mounting semiconductor die on said die mount portions and wire bonding said die to said lead members;   (e) plastic encapsulating said semiconductor devices; and   (f) shearing away said joining means to produce individual semiconductor devices each having a die mount portion which is electrically isolated from its device leads.   
     
     
       2. A semiconductor device lead frame comprising: (a) first preassembly structure means including; (1) a plurality of die mount portions each having an upper surface;   (2) first tie bar means coupled to said die mount portions for holding said die mount portions in fixed relationship with each other and for holding said upper surfaces in substantially a first plane;   (3) first joining means coupled to said first tie bar means, said first joining means interspersed between said die mount portions and said first joining means terminating in a first attachment region; and     (b) second preassembly structure means including: (1) a plurality of groups of separated lead members;   (2) second tie bar means joining said separated lead members and said groups of separated lead members; and   (3) second joining means coupled to said second tie bar means, said second joining means interspersed between said groups and said second joining means terminating in a second attachment region;     (c) wherein said first preassembly structure and said second preassembly structure are held in fixed relationship with respect to each other by joining said first and said second attachment regions.   
     
     
       3. A semiconductor device lead frame as recited in claim 2 wherein said first attachment region of said first preassembly structure is an extruded post and said second attachment region of said second preassembly structure is a flat region including a hole adapted to receive said extruded post such that such joining comprises inserting said extruded post through said hole and riveting to form a rigid structure. 
     
     
       4. A semiconductor device lead frame as recited in claim 3 wherein said first joining means includes an offset bend which positions said first attachment region in a plane above said first plane such that said groups of separated lead members are positioned above said upper surface of said die mount portions after said joining of said first and said second preassembly structures. 
     
     
       5. A semiconductor device lead frame as recited in claim 3 wherein said first joining means and said second joining means are positioned between said die mount portions and between said groups of separated lead members such that said joining means can be sheared away after completion of assembly of the semiconductor device. 
     
     
       6. A semiconductor device lead frame comprising: (a) first preassembly structure means including, (1) a plurality of die mount portions,   (2) first tie bar means coupled to said die mount portions for holding said die mount portions in fixed relationship with each other,   (3) first joining means interspersed between said die mount portions and terminating in first attachment regions,     (b) second preassembly structure means including: (1) a plurality of groups of separated lead members,   (2) second tie bar means interconnecting said separated lead members and said groups of separated lead members,   (3) second joining means interspersed between said groups and terminating in second attachment regions,     (c) wherein said first preassembly structure and said second preassembly structure are held in fixed relationship with respect to each other by joining said first and said second attachment regions.